The invention resides in an apparatus and a method for producing light diffraction-active structures on workpieces by way of embossing or stamping.
For generating colorful optical effects, such as holograms or similar on metal surfaces, for example on coins, it is known to provide the surface of the particular object with a diffraction-active surface structure. Such a surface structure causes light interferences which generate the desired optical effect. Diffraction-active surfaces are not provided only for decoration purposes, but they may also prevent falsifications. The optical effect is based on very fine, periodic relief-like lattice structures which are embossed into the respective surface. The best known example is found in metallized plastic foils with holograms used on charge cards and money bills and on seals. While a smooth surface reflects incident angles, different reflection angles are obtained in connection with diffraction lattice structures which results for example on hologram foils. The manufacture of diffraction-active foils is based on the embossing of the thermoplastic materials by galvanically formed nickel stamps and subsequent metallization of the lattice surface. In addition, there are other methods by which diffraction lattice structures can be applied directly to the surfaces of metallic bodies.
WO 2004/045866 A1 discloses for example a way for generating such surface structures on metal surfaces of workpieces by means of an embossing stamp. The stamp includes a lattice and/or line structure which is formed onto the workpiece during the embossing of the workpiece. Herein, the embossment stamp is formed by a monocrystalline diamond which is soldered by cobalt to a substrate body. The negative structure of the infraction lattice structure to be formed is machined into the embossment stamp by means of a laser. In addition, the embossment stamp may be polished by an electron beam or x-ray treatment.
With this process however, the size of the interference active structures to be generated is limited to the size of the diamonds that are available.
In addition, DE 100 02 644 A1 discloses the manufacture of diffraction active relief structures on coin surfaces. To this end, the respective semi-finished workpiece is first coded in an immersion bath with a thin light-sensitive plastic foil. Then the plastic foil is exposed to light and is developed. In this way, microscopically fine spots on the coin surface are exposed. In an etching bath, thin microscopic relief structures are then etched into the coin surface, whereupon the light sensitive plastic is again removed. The DE 100 644 A1 discloses furthermore a concept for a counterfeit-proof coin with diffraction optical features which complement the conventional design elements of a coin. These should be visually recognizable but also machine-readable, wherein the machine recognition of the infraction optical signature is based on the deformation of the spatial position of the diffraction maxima. A reading apparatus is presented which is installed in coin examination devices and similar apparatus. The core of this reading apparatus is a light source and a sensor arrangement tuned thereto by which it is determined, among others, whether the diffraction maxima of the light reflected from the signature on the coin occur at the location as expected based on the wavelength of the light and the lattice parameters of the structure.
Further, DE 197 22 575 A1 discloses the manufacture of interference-capable microstructures, for example, for the generation of holograms on coins or such structures by means of embossing stamps. In this method, several hard layers consisting for example of cobalt- or nickel alloys are deposited into which diamond crystals are embedded. The layers may be deposited by special alloy both immersion bathes and metal spray processes, by galvanic deposition or by plasma deposition. The surface layer is formed by a diamond-like layer, which contains the microstructure. For generating the microstructures, the above reference refers generally to galvanic processes, electronic or laser engraving, fine etching techniques and direct engraving by diamond tools.
But it is still difficult on one hand, to produce macroscopic relief structures as they are common in connection with coins and, on the other hand, diffraction structures.
It is therefore the object of the present invention to provide a practical method and a corresponding arrangement for the technical manufacture of diffraction structures by embossing techniques as well as relief structures on metal surfaces. It is particularly an object of the invention to provide a practically useable cost effective manufacturing procedure for diffractograms embossed into metal for use in the mass production of circulation coins and tokens or, respectively, value stamps, but also for the manufacture of special coins, medallions, and similar items.